Organization of the efferent projections from the spinal cervical enlargement to the parabrachial area and periaqueductal gray: a PHA-L study in the rat

Neuronal circuitry for pain processing in the dorsal horn

Neurons in the spinal dorsal horn process sensory information, which is then transmitted to several brain regions, including those responsible for pain perception. The dorsal horn provides numerous potential targets for the development of novel analgesics and is thought to undergo changes that contribute to the exaggerated pain felt after nerve injury and inflammation. Despite its obvious importance, we still know little about the neuronal circuits that process sensory information, mainly because of the heterogeneity of the various neuronal components that make up these circuits. Recent studies have begun to shed light on the neuronal organization and circuitry of this complex region.

Effect of chronic pain on morphine-induced respiratory depression in mice

Respiratory depression is the most well-known and dangerous side-effect of opioid analgesics. Clinical investigations have revealed that this opioid-induced respiratory depression is less severe in patients with chronic pain, but the mechanisms that underlie this phenomenon are unknown. Therefore, the present study was designed to examine the influence of chronic pain on morphine-induced respiratory depression. Respiration was detected by double-chamber, flow-through whole-body plethysmography. Respiratory frequency was dose-dependently and significantly decreased after morphine administration. This effect peaked at 30 min after administration and lasted 3 h. In contrast, tidal volume was increased. Minute volume was significantly decreased by morphine at a higher dose, but not a lower dose. In nerve-ligated mice, a morphine-induced decrease in respiratory frequency was observed, whereas the increase of tidal volume was more prominent. A decrease in minute volume was not observed in nerve-ligated mice. This attenuation of the morphine-induced decrease in minute volume in nerve-ligated mice was reversed by treatment with the serotonin (5-HT)4a receptor antagonist GR125487. Moreover, treatment with the 5-HT4 receptor agonist mosapride antagonized the morphine-induced decrease in minute volume, due to the enhancement of tidal volume. Finally, the expression of 5-HT4a receptor in the brainstem was enhanced in nerve-ligated mice compared to that in sham-operated mice. These results suggest that the decrease in morphine-induced respiratory depression under chronic pain is mediated by the enhancement of 5-HT4a receptor systems in the brainstem.

Neural substrate of depression during migraine

Migraine headache is triggered by and associated with a variety of hormonal, emotional, nutritional and physiological changes. The perception of migraine headache is formed when nociceptive signals originating in the meninges are conveyed to the somatosensory cortex through the trigeminal ganglion, medullary dorsal horn and thalamus. We propose that different migraine triggers activate a wide variety of brain areas that impinge on parasympathetic neurons innervating the meninges. According to this hypothesis, migraine triggers such as stress activate multiple hypothalamic, limbic and cortical areas, all of which contain neurons that project to the preganglionic parasympathetic neurons in the superior salivatory nucleus (SSN). The SSN, in turn, activates postganglionic parasympathetic neurons in the sphenopalatine ganglion, resulting in vasodilation and local release of inflammatory molecules that activate meningeal nociceptors. We propose that trigeminovascular projections from the medullary dorsal horn to selective areas in the midbrain, hypothalamus, amygdala and basal forebrain are functionally positioned to produce migraine symptoms such as irritability, loss of appetite, fatigue, depression and the quest for solitude. The network of bidirectional trafficking by which the trigeminovascular system can activate the same brain areas that have triggered its own activity in the first place provides an attractive mechanism of perpetual feedback that drives a migraine attack for many hours and even days.

A quantitative study of brainstem projections from lamina I neurons in the cervical and lumbar enlargement of the rat

Lamina I of the rat spinal cord contains neurons that project to various brain areas including thalamus, periaqueductal grey matter (PAG), lateral parabrachial area (LPb), caudal ventrolateral medulla and a region in dorsal medulla that includes the nucleus tractus solitarius and dorsal reticular nucleus. We have shown that spinothalamic lamina I neurons are infrequent in rat lumbar enlargement, where they constitute approximately 5% of the estimated 400 projection neurons on each side of the L4 segment (Al-Khater and Todd, 2009). They are more numerous in cervical enlargement, but the total number of lamina I projection neurons in this region was not known. Here we have used paired injections of retrograde tracers into the brainstem to estimate the number of lamina I projection cells in the C7 segment. Our results suggest that there are approximately 215 lamina I projection cells per side, and that spinothalamic cells therefore make up approximately 42% of this population. The proportion of lamina I projection neurons labelled from PAG is higher in cervical than lumbar enlargement, while the proportion labelled from dorsal medulla is similar in the two regions. We also found that lamina I cells in L4 that project to the dorsal medulla are included in the population retrogradely labelled from LPb, thus confirming the estimate that there are around 400 lamina I projection cells in this segment.

Soma size distinguishes projection neurons from neurokinin 1 receptor-expressing interneurons in lamina I of the rat lumbar spinal dorsal horn

Lamina I of the spinal dorsal horn contains neurons that project to various brain regions, and ∼80% of these projection cells express the neurokinin 1 receptor (NK1r), the main receptor for substance P. Two populations of NK1r-immunoreactive neurons have been identified in lamina I: small weakly immunoreactive cells and large cells with strong immunolabelling [Cheunsuang O and Morris R (2000) Neuroscience 97:335–345]. The main aim of this study was to test the hypothesis that the large cells are projection neurons and that the small cells are interneurons. Projection neurons were identified by injection of tracers into the caudal ventrolateral medulla and lateral parabrachial area, and this was combined with immunostaining for NK1r. We found a bimodal size distribution for NK1r-immunoreactive neurons. The small cells (with somatic cross-sectional areas <200 μm²) showed weak immunoreactivity, while immunostaining intensity was variable among the large cells. Virtually all (99%) of the immunoreactive cells with soma areas >200 μm² were retrogradely labelled, while only 10% of retrogradely labelled cells were smaller than this. Soma sizes of retrogradely labelled neurons that lacked NK1r did not differ from those of NK1r-expressing projection neurons. It has been suggested that a population of small pyramidal projection neurons that lack NK1r may correspond to cells activated by innocuous cooling, and we therefore assessed the morphology of retrogradely labelled cells that were not NK1r-immunoreactive. Fifteen percent of these were pyramidal, but these did not differ in size from pyramidal NK1r-immunoreactive projection neurons. These results confirm that large NK1r-immunoreactive lamina I neurons are projection cells, and suggest that the small cells are interneurons. Since almost all of the NK1r-immunoreactive cells with soma size >200 μm² were retrogradely labelled, cells of this type can be identified as projection cells in anatomical studies.

Bilateral phosphorylation of ERK in the lateral and centrolateral amygdala during unilateral storage of fear memories

We previously showed that when rats were trained to fear an auditory conditioned stimulus (CS) by pairing it with a mild unilateral shock to the eyelid (the unconditioned stimulus, or US), conditioned freezing depended upon the amygdala contralateral but not ipsilateral from the US. It was proposed that convergent activation of amygdala neurons by the CS and US occurred mainly in the amygdala contralateral from US delivery, causing memories of the CS-US association to be stored primarily by that hemisphere. In the present study, we further tested this interpretation by administering unilateral infusions of U0126 (in 50% dimethyl sulfoxide (DMSO) vehicle) to block phosphorylation of extracellular signal-responsive kinase (ERK) in the amygdala prior to CS-US pairings. Conditioned freezing was impaired 24 h after training when U0126 was infused contralaterally-but not ipsilaterally-from the US, suggesting that fear memories were consolidated mainly by the contralateral amygdala. However, immunostaining experiments revealed that ERK phosphorylation was elevated in both hemispheres of the amygdale's lateral (LA) and centrolateral (CeL) nuclei after paired (but not unpaired (UNP)) presentations of the CS and US. Thus, fear acquisition induced ERK phosphorylation bilaterally in the amygdala, even though the ipsilateral hemisphere did not appear to participate in conditioned freezing. These findings suggest that associative plasticity may occur in both amygdala hemispheres even when only one hemisphere is involved in freezing behavior. Conditioning-induced ERK phosphorylation was identical in both hemispheres of LA, but was slightly greater in the contralateral than ipsilateral hemisphere of CeL. Hence, asymmetric induction of plasticity in CeL might help to explain why conditioned freezing depends preferentially upon the amygdala contralateral from the US in our fear conditioning paradigm.

Freezing of enkephalinergic functions by multiple noxious foci: a source of pain sensitization?

Background

The functional significance of proenkephalin systems in processing pain remains an open question and indeed is puzzling. For example, a noxious mechanical stimulus does not alter the release of Met-enkephalin-like material (MELM) from segments of the spinal cord related to the stimulated area of the body, but does increase its release from other segments.

Methodology/Principal Findings

Here we show that, in the rat, a noxious mechanical stimulus applied to either the right or the left hind paw elicits a marked increase of MELM release during perifusion of either the whole spinal cord or the cervico-trigeminal area. However, these stimulatory effects were not additive and indeed, disappeared completely when the right and left paws were stimulated simultaneously.

Conclusion/Significance

We have concluded that in addition to the concept of a diffuse control of the transmission of nociceptive signals through the dorsal horn, there is a diffuse control of the modulation of this transmission. The “freezing” of Met-enkephalinergic functions represents a potential source of central sensitization in the spinal cord, notably in clinical situations involving multiple painful foci, e.g. cancer with metastases, poly-traumatism or rheumatoid arthritis.

Collateral projections of neurons in laminae I, III, and IV of rat spinal cord to thalamus, periaqueductal gray matter, and lateral parabrachial area

Projection neurons in lamina I, together with those in laminae III–IV that express the neurokinin 1 receptor (NK1r), form a major route through which nociceptive information reaches the brain. Axons of these cells innervate various targets, including thalamus, periaqueductal gray matter (PAG), and lateral parabrachial area (LPb), and many cells project to more than one target. The aims of this study were to quantify projections from cervical enlargement to PAG and LPb, to determine the proportion of spinothalamic neurons at lumbar and cervical levels that were labelled from PAG and LPb, and to investigate morphological differences between projection populations. The C7 segment contained fewer lamina I spinoparabrachial cells than L4, but a similar number of spino-PAG cells. Virtually all spinothalamic lamina I neurons at both levels were labelled from LPb and between one-third and one-half from PAG. This suggests that significant numbers project to all three targets. Spinothalamic lamina I neurons differed from those labelled only from LPb in that they were generally larger, were more often multipolar, and (in cervical enlargement) had stronger NK1r immunoreactivity. Most lamina III/IV NK1r cells at both levels projected to LPb, but few were labelled from PAG. The great majority of these cells in C7 and over one-fourth of those in L4 were spinothalamic, and at each level some projected to both thalamus and LPb. These results confirm that neurons in these laminae have extensive collateral projections and suggest that different neuronal subpopulations in lamina I have characteristic patterns of supraspinal projection. J. Comp. Neurol. 515:629–646, 2009.

Inputs to serotonergic neurons revealed by conditional viral transneuronal tracing

Descending projections arising from brainstem serotonergic (5HT) neurons contribute to both facilitatory and inhibitory controls of spinal cord "pain" transmission neurons. Unclear, however, are the brainstem networks that influence the output of these 5HT neurons. To address this question, here we used a novel neuroanatomical tracing method in a transgenic line of mice in which Cre recombinase is selectively expressed in 5HT neurons (ePet-Cre mice). Specifically, we injected the conditional pseudorabies virus recombinant (BA2001) that can replicate only in Cre-expressing neurons. Because BA2001 transports exclusively in a retrograde manner, we were able to reveal a subset of the neurons and circuits that are located upstream of the Cre-expressing 5HT neurons. We show that diverse brainstem regions differentially target the 5HT neurons of the dorsal raphe (DR) and the nucleus raphe magnus of the rostroventral medulla (RVM). Among these are several catecholaminergic and cholinergic cell groups, the periaqueductal gray, several brainstem reticular nuclei, and the nucleus of the solitary tract. We conclude that a brainstem 5HT network integrates somatic and visceral inputs arising from various areas of the body. We also identified a circuit that arises from projection neurons of deep spinal cord laminae V-VIII and targets the 5HT neurons of the NRM, but not of the DR. This spinoreticular pathway constitutes an anatomical substrate through which a noxious stimulus can activate 5HT neurons of the NRM and in turn could trigger descending serotonergic antinociceptive controls.

Sensitization of lamina I spinoparabrachial neurons parallels heat hyperalgesia in the chronic constriction injury model of neuropathic pain

It has been proposed that spinal lamina I neurons with ascending axons that project to the midbrain play a crucial role in hyperalgesia. To test this hypothesis the quantitative properties of lamina I spinoparabrachial neurons in the chronic constriction injury (CCI) model of neuropathic pain were compared to those of unoperated and sham-operated controls. Behavioural testing showed that animals with a CCI exhibited heat hyperalgesia within 4 days of the injury, and this hyperalgesia persisted throughout the 14-day post-operative testing period. In the CCI, nociceptive lamina I spinoparabrachial neurons had heat thresholds that were significantly lower than controls (43.0 ± 2.8°C vs. 46.7 ± 2.6°C; P < 10⁻⁴, ANOVA). Nociceptive lamina I spinoparabrachial neurons were also significantly more responsive to graded heat stimuli in the CCI, compared to controls (P < 0.02, 2-factor repeated-measures ANOVA), and increased after-discharges were also observed. Furthermore, the heat-evoked stimulus–response functions of lamina I spinoparabrachial neurons in CCI animals co-varied significantly (P < 0.03, ANCOVA) with the amplitude of heat hyperalgesia determined behaviourally. Taken together these results are consistent with the hypothesis that lamina I spinoparabrachial neurons have an important mechanistic role in the pathophysiology of neuropathic pain.

Brainstem and thalamic projections from a craniovascular sensory nervous centre in the rostral cervical spinal dorsal horn of rats

To examine the ascending projections from the headache-related trigeminocervical complex in rats, biotinylated dextran amine (BDA) was injected into the ventrolateral dorsal horn of segments C1 and C2, a region previously demonstrated to receive input from sensory nerves in cranial blood vessels. Following injections into laminae I-II, BDA-labelled terminations were found bilaterally in several nuclei in the pons and the midbrain, including the pontine reticular nucleus, the parabrachial nuclei, the cuneiform nucleus and the periaqueductal grey. In the diencephalon, terminations were confined to the contralateral side and evident foremost in the posterior nuclear group, especially its triangular part, and in the ventral posteromedial nucleus. Following injections extending through laminae I-IV, anterograde labelling was more extensive. Some of the above regions are likely to be involved in the central processing of noxious signals of craniovascular origin and therefore putatively involved in mechanisms associated with primary headaches.

Characterization of parabrachial subnuclei in mice with regard to salt tastants: possible independence of taste relay from visceral processing

We examined whether salt taste and/or abdominal illness were dealt within different subnuclei in the parabrachial nucleus (PBN) in mice, using retrograde tracing methods and c-Fos-like immunoreactivity (FLI) detection procedures. Some PBN subnuclei have distinct functions and receive various sensory inputs from the nucleus of the solitary tract (NTS) and other areas and relay them to the higher order nuclei such as the thalamus. The afferent-dependent pattern of FLI has been investigated in the PBN. However, it is unclear in which PBN subnuclei the tastants induce c-Fos, or whether PBN subnuclei process taste inputs separately from other inputs, or integrate them. After the tracer injections into the thalamic taste relay, the retrograde labeled cells revealed the taste relay cells in the PBN at the boundary with the superior cerebellar peduncle of both the inner part of the external lateral subnucleus and the medial subnucleus and in the waist area. On the other hand, NaCl intake induced intense FLI in the dorsal lateral subnucleus, whereas LiCl intake yielded intense FLI in both the dorsal lateral subnucleus and the outer part of the external lateral subnucleus. Thus, the present findings that subnuclei relaying taste information to the thalamus do not yield FLI in response to salt taste and abdominal illness indicate that they lack FLI yielding pathways or that they are independent from the subnuclei processing salt taste and visceral information via c-Fos in mice.

Central nervous system networks involved in the processing of meningeal and cutaneous inputs from the ophthalmic branch of the trigeminal nerve in the rat

This study analysed the organization of central nervous system networks involved in the processing of meningeal inputs in the male, Sprague-Dawley rat. We injected the anterograde tracer, biotin dextran, into areas of the medullary trigeminal nucleus caudalis (Sp5C), which receive inputs from the ophthalmic division of the trigeminal nerve. Double-labelling immunohistochemical studies were then performed to compare calcitonin gene-related peptide (CGRP) or serotonin 1D (5HT1(D)) receptor distributions in the areas innervated by Sp5C neurons. Dense, topographically organized intratrigeminal connections were observed. Sp5C neurons projected to the commissural subnucleus of the solitary tract, A5 cell group region/superior salivatory nucleus, lateral periaqueductal grey matter, inferior colliculus and parabrachial nuclei. Trigeminothalamic afferents were restricted to the posterior group and ventroposteromedial thalamic nuclei. Some of these areas are also immunoreactive for 5HT1(D) and CGRP and thus remain potential central targets of triptan molecules and other antimigraine drugs.

Central control of thermogenesis in mammals

Thermogenesis, the production of heat energy, is an essential component of the homeostatic repertoire to maintain body temperature in mammals and birds during the challenge of low environmental temperature and plays a key role in elevating body temperature during the febrile response to infection. The primary sources of neurally regulated metabolic heat production are mitochondrial oxidation in brown adipose tissue, increases in heart rate and shivering in skeletal muscle. Thermogenesis is regulated in each of these tissues by parallel networks in the central nervous system, which respond to feedforward afferent signals from cutaneous and core body thermoreceptors and to feedback signals from brain thermosensitive neurons to activate the appropriate sympathetic and somatic efferents. This review summarizes the research leading to a model of the feedforward reflex pathway through which environmental cold stimulates thermogenesis and discusses the influence on this thermoregulatory network of the pyrogenic mediator, prostaglandin E(2), to increase body temperature. The cold thermal afferent circuit from cutaneous thermal receptors ascends via second-order thermosensory neurons in the dorsal horn of the spinal cord to activate neurons in the lateral parabrachial nucleus, which drive GABAergic interneurons in the preoptic area to inhibit warm-sensitive, inhibitory output neurons of the preoptic area. The resulting disinhibition of thermogenesis-promoting neurons in the dorsomedial hypothalamus and possibly of sympathetic and somatic premotor neurons in the rostral ventromedial medulla, including the raphe pallidus, activates excitatory inputs to spinal sympathetic and somatic motor circuits to drive thermogenesis.

A thermosensory pathway that controls body temperature

Defending body temperature against environmental thermal challenges is one of the most fundamental homeostatic functions that are governed by the nervous system. Here we describe a somatosensory pathway that essentially constitutes the afferent arm of the thermoregulatory reflex that is triggered by cutaneous sensation of environmental temperature changes. Using in vivo electrophysiological and anatomical approaches in the rat, we found that lateral parabrachial neurons are pivotal in this pathway by glutamatergically transmitting cutaneous thermosensory signals received from spinal somatosensory neurons directly to the thermoregulatory command center, the preoptic area. This feedforward pathway mediates not only sympathetic and shivering thermogenic responses but also metabolic and cardiac responses to skin cooling challenges. Notably, this 'thermoregulatory afferent' pathway exists in parallel with the spinothalamocortical somatosensory pathway that mediates temperature perception. These findings make an important contribution to our understanding of both the somatosensory system and thermal homeostasis -- two mechanisms that are fundamental to the nervous system and to our survival.

Morphological characterization of spinal cord dorsal horn lamina I neurons projecting to the parabrachial nucleus in the rat

Many Rexed's lamina I neurons are nociceptive and project to the brain. Lamina I projection neurons can be classified as multipolar, fusiform, or pyramidal, based on cell body shape and characteristics of their proximal dendrites in the horizontal plane. There is also evidence that both multipolar and fusiform cells are nociceptive and pyramidal neurons nonnociceptive. In this investigation we identified which types of lamina I neurons belong to the spinoparabrachial tract in the rat and characterized them regarding the presence or absence of neurokinin-1 receptor (NK-1r) immunoreactivity. For this, cholera toxin subunit B (CTb), conjugated to a fluorescent marker was injected unilaterally into the parabrachial nucleus. Sections were additionally stained for the detection of NK-1r immunoreactivity and were examined using fluorescence and confocal microscopy. Serial confocal optical sections and 3D reconstructions were obtained for a considerable number of neurons per animal. Using immunofluorescence, we assessed the proportion of lamina I neurons belonging to the spinoparabrachial (SPB) tract and/or expressing NK-1r. The relative distribution of neurons belonging to the SPB tract was: 38.7% multipolar, 36.8% fusiform, 22.7% pyramidal, and 1.9% unclassified. Most of the SPB neurons expressing NK-1r were either multipolar or fusiform. Pyramidal SPB neurons were seldom immunoreactive for NK-1r, an observation that provides further support to the concept that most lamina I projection neurons of the pyramidal type are nonnociceptive. In addition, our study provides further evidence that these distinct morphological types of neurons differ in their phenotypic properties, but not in their projection patterns.

Expression of c-Fos in the parabrachial nucleus following peripheral nerve injury in rats

Chronic constriction injury (CCI) of the sciatic nerve in rats evokes c-Fos expression at spinal cord level. Using immunohistochemical methods we studied changes in c-Fos expression in the brain stem area, which is suggested as one of the major targets of projection neurons in the superficial dorsal horn laminae, i.e., the parabrachial area. During the first week following injury, the animals developed tactile allodynia. At this time we found an increase of c-Fos positive neurons in the parabrachial area, mainly in the pontine part where the group of c-Fos immunoreactive neurons was present in the dorsal part of lateral parabrachial subnuclei. The number of c-Fos positive neurons gradually decreased up to 14 days following CCI. The specific activation of brain stem neurons during onset of mechanical allodynia could underlie the changes in central nociceptive processing following peripheral nerve injury.

Learned preferences induced by electrical stimulation of a food-related area of the parabrachial complex: Effects of naloxone

Electrical stimulation of the External Lateral Parabrachial Subnucleus (LPBe), a food-related area, induced behavioral preferences for associated stimuli in a taste discrimination learning task. Although this stimulation appeared to be ineffective to elicit standard lever press self-stimulation, it induced place preference for one of two training compartments of a rectangular maze in which animals (adult male Wistar rats) received concurrent electrical brain stimulation. In subjects that consistently showed a preference behavior in different trials, administration of the opioid antagonist naloxone (4 mg/ml/kg) blocked concurrent learning when the test was made in a new maze but not in the same maze in which animals had learned the task. These results are discussed in terms of the possible participation of the LPBe subnucleus in different natural and artificial brain reward systems.

Brain stem afferent connections of the amygdala in the rat with special references to a projection from the parabigeminal nucleus: a fluorescent retrograde tracing study

A recently revealed important function of the amygdala (Am) is that it acts as the brain's "lighthouse", which constantly monitors the environment for stimuli which signal a threat to the organism. The data from patients with extensive lesions of the striate cortex indicate that "unseen" fearful and fear-conditioned faces elicit increased Am responses. Thus, also extrageniculostriate pathways are involved. A multisynaptic pathway from the retina to the Am via the superior colliculus (SC) and the pulvinar was recently suggested. We here present data based on retrograde neuronal labeling following injection of the fluorescent tracer Fluoro-Gold in the rat Am that the parabigeminal nucleus (Pbg) emits a substantial, bilateral projection to the Am. This small cholinergic nucleus (Ch8 group) in the midbrain tegmentum is a subcortical relay visual center that is reciprocally connected with the SC. We suggest the existence of a second extrageniculostriate multisynaptic connection to Am: retina-SC-Pbg-Am, that might be very effective since all tracts listed above are bilateral. In addition, we present hodological details on other brainstem afferent connections of the Am, some of which are only recently described, and some others that still remain equivocal. Following selective injections of Fluoro-Gold in the Am, retrogradely labeled neurons were observed in parasubthalamic nucleus, peripeduncular nucleus, periaqueductal gray, dopaminergic nuclear complex (substantia nigra pars lateralis and pars compacta, paranigral, parabrachial pigmented and interfascicular nuclei, rostral and caudal linear nuclei, retrorubral area), deep mesencephalic nucleus, serotoninergic structures (dorsal, median and pontine raphe nuclei), laterodorsal and pedunculopontine tegmental nuclei (Ch6 and Ch5 groups), parabrachial nuclear complex, locus coeruleus, nucleus incertus, ventrolateral pontine tegmentum (A5 group), dorsomedial medulla (nucleus of the solitary tract, A2 group), ventrolateral medulla (A1/C1 group), and pars caudalis of the spinal trigeminal nucleus. A bilateral labeling of the upper cervical spinal cord was also observed.

Unitary hypothesis for multiple triggers of the pain and strain of migraine

Migraine headache is triggered by and associated with a variety of hormonal, emotional, nutritional, and physiological changes. The perception of migraine headache is formed when nociceptive signals originating in the meninges are conveyed to the somatosensory cortex through the trigeminal ganglion, medullary dorsal horn, and thalamus. Is there a common descending pathway accounting for the activation of meningeal nociceptors by different migraine triggers? We propose that different migraine triggers activate a wide variety of brain areas that impinge on parasympathetic neurons innervating the meninges. According to this hypothesis, migraine triggers such as perfume, stress, or awakening activate multiple hypothalamic, limbic, and cortical areas, all of which contain neurons that project to the preganglionic parasympathetic neurons in the superior salivatory nucleus (SSN). The SSN, in turn, activates postganglionic parasympathetic neurons in the sphenopalatine ganglion, resulting in vasodilation and local release of inflammatory molecules that activate meningeal nociceptors. Are there ascending pathways through which the trigeminovascular system can induce the wide variety of migraine symptoms? We propose that trigeminovascular projections from the medullary dorsal horn to selective areas in the midbrain, hypothalamus, amygdala, and basal forebrain are functionally positioned to produce migraine symptoms such as irritability, loss of appetite, fatigue, depression, or the quest for solitude. Bidirectional trafficking by which the trigeminovascular system can activate the same brain areas that have triggered its own activity in the first place provides an attractive network of perpetual feedback that drives a migraine attack for many hours and even days.

Unilateral storage of fear memories by the amygdala

Pavlovian fear conditioning is an associative learning task in which subjects are trained to respond defensively to a neutral conditioned stimulus (CS) by pairing it with an aversive unconditioned stimulus (US). This type of learning depends critically on the amygdala, and evidence suggests that synaptic plasticity within the lateral nucleus of the amygdala (LA) may be responsible for storing memories of the CS-US association. In the present study, we trained rats to fear an auditory CS by pairing it with a shock US delivered to one eyelid. Conditioning was assessed by measuring freezing responses evoked by the CS during a subsequent test session. The amygdala was unilaterally inactivated during either the training or the testing session by intracranial infusions of muscimol into the LA. We found that both acquisition and expression of conditioned freezing to the CS depended on the amygdala contralateral but not ipsilateral from the eyelid where the shock US was delivered. To explain this surprising result, we propose that the shock US is relayed from the eyelid to the amygdala via lateralized nociceptive sensory pathways, which causes memories of the CS-US association to be stored by the amygdala contralateral but not ipsilateral from the shocked eyelid. Our results demonstrate that the fear-learning circuitry of the amygdala is functionally lateralized according to the anatomical source of predicted threats. In future studies, the cellular mechanisms of emotional memory storage might be pinpointed by identifying cellular processes that occur only in the amygdala contralateral but not ipsilateral from the US during lateralized fear conditioning.

Branching patterns of parabrachial neurons projecting to the central extended amgydala: single axonal reconstructions

Electrophysiological evidence suggests that the spinoparabrachioamygdaloid pathway carries nociceptive information that may be important for the elaboration of physiological and emotional responses to noxious events. The pontine parabrachial nucleus (pPB) sends a massive projection to the central nucleus of the amygdala (CeA) and lateral bed nucleus of the stria terminalis (BSTL), both regions belonging to a broader macrostructure, the central extended amygdala (EAc). The aim of this study was to examine whether different EAc components are targeted by a same pPB neuron, by reconstructing single axonal branching patterns after anterograde labelling. Small deposits of biotinylated dextran amine in the region of the external lateral pPB result in dense and specific labelling in the whole EAc. Reconstructed axons innervate either the lateral or the capsular part of the CeA with perisomatic or bushy terminals, respectively. A subset of axons enters the stria terminalis rostrally to follow its trajectory caudally toward the CeA. Individual axons targeting the CeA usually send collaterals to other EAc components, especially those projecting to the lateral CeA, which often coinnervate the BSTL. By contrast, only few branches were found outside the EAc. These results suggest that the noxious information travelling from the pPB to the CeA may also be transmitted to other EAc components. This pPB-EAc pathway, which appears distinct from the parabrachiohypothalamic and parabrachiothalamic projections, would be the anatomical basis through which the EAc elaborates the autonomic, endocrine, and emotional components of pain.

Enkephalinergic afferents of the centromedial amygdala in the rat

The connectivity of the amygdaloid complex has been extensively explored with both anterograde and retrograde tracers. Even though the afferents of the centromedial amygdala [comprising the central (CEA) and medial (MEA) amygdaloid nuclei] are well established, relatively little is known about the neuropeptide phenotype of these connections. In this study, we first examined the distribution of mu-opioid receptor (MOR) and delta-opioid receptor (DOR) in the amygdala via in situ hybridization and immunohistochemistry. We then investigated the distribution of Met-enkephalin (ENK) and Leu-ENK fibers with immunohistochemistry and examined the distribution of preproenkephalin mRNA in the amygdala by using in situ hybridization. Finally, we examined the ENK projections to the CEA and MEA by using stereotaxic injections of the retrograde tracer cholera toxin subunit B or fluorogold revealed by immunohistochemistry combined with in situ hybridization to identify ENKergic neurons. Our results indicate that the centromedial amygdala receives ENK afferents, as indicated by the presence of MOR, DOR, and ENK fibers in the CEA and MEA, originating primarily from the bed nucleus of the stria terminalis (BST) and from other amygdaloid nuclei. The posterior BST, the basomedial nucleus (BMA), and the cortical nucleus of the amygdala (COA) were found to be the major ENK afferents of the MEA, whereas the anterolateral BST, the COA, the MEA, and the BMA provided the main ENKergic innervation of the CEA. In addition, we found that the ventromedial nucleus of the hypothalamus and the pontine parabrachial nucleus provide a moderate ENK input to the CEA and MEA. The functional implications of these connections in stress, anxiety, and nociception are discussed.

Respiratory rhythm entrainment by somatic afferent stimulation

Respiratory and locomotor patterns are coupled during locomotion. The objectives of this study were to (1) demonstrate that respiratory rhythms are entrained by sensory input from somatic afferents, (2) establish whether the parabrachial nucleus mediates entrainment, (3) examine responses of single respiratory neurons in the ventral respiratory group (VRG) to somatic afferent stimulation, and (4) use a computational model of the pontomedullary respiratory network (Rybak et al., 2004a,b) to suggest neuronal mechanisms for entrainment. We used an in situ preparation in young rats that retained pontomedullary respiratory circuits and spinal pathways transmitting somatosensory input. We demonstrate that rhythmic stimulation of somatic afferents entrains respiratory rhythm on a 1:1 basis (1:1), increasing breathing frequency up to approximately 1.4-2.2 times greater than spontaneous frequency. Stable entrainment occurred only when stimuli were delivered during expiration. Reversible blockade of the lateral parabrachial nucleus eliminated entrainment. Somatic afferent stimulation produced significant increases in the firing rate of augmenting expiratory (E2) neurons but shortened the firing duration of postinspiratory (post-I) neurons. A computational model reproduced 1:1 entrainment and other experimental findings based on the assumption that the somatic afferents initiate early onset of inspiration via activation of medullary E2 neurons. The model also predicted that afferent stimulation evoked transient hyperpolarization of ramp-inspiratory (ramp-I) neurons. This was confirmed experimentally by intracellular recording from ramp-I neurons. Our experimental and modeling results demonstrate that an entrainment pathway from somatic afferents to the VRG via the lateral parabrachial nucleus causes resetting of respiratory rhythm through excitation of E2 and consequent inhibition of post-I neurons.

Neurokininergic mechanism within the lateral crescent nucleus of the parabrachial complex participates in the heart-rate response to nociception

We wanted to ascertain whether the lateral parabrachial nucleus was involved in mediating the heart-rate response evoked during stimulation of somatic nociceptors. Reversible inactivation of the lateral parabrachial nucleus, using a GABA(A) agonist, reduced the reflex tachycardia evoked during noxious (mechanical) stimulation of the forelimb by approximately 50%. The same effect was observed after blockade of neurokinin 1 receptors within the lateral parabrachial nucleus, indicating a possible involvement for substance P as a neurotransmitter. Immunocytochemistry revealed a strong expression of substance P-immunoreactive fibers and boutons in all lateral subnuclei, but they were particularly dense in the lateral crescent subnucleus. Histological verification showed that the most effective injection sites for attenuating the noxious-evoked tachycardia were all placed in or near to the lateral crescent nucleus of the lateral parabrachial complex. Many single units recorded from this region were activated by high-intensity brachial nerve stimulation. The brachial nerve evoked firing responses of some of these neurons was reversibly reduced after local delivery of a neurokinin 1 receptor antagonist. However, only a minority of these neurons followed a paired-pulse stimulation protocol applied to the spinal cord, suggesting a predominance of indirect projections from the spinal cord to the parabrachial nucleus. We conclude that the cardiac component of the response to somatic nociception involves indirect spinal pathways that most likely excite neurons located in the lateral crescent nucleus of the parabrachial complex via activation of neurokinin 1 receptors.

Synaptic input of rat spinal lamina I projection and unidentified neurones in vitro

Spinal lamina I projection neurones that transmit nociceptive information to the brain play a pivotal role in hyperalgesia in various animal models of inflammatory and neuropathic pain. Consistently, activity-dependent long-term potentiation can be induced at synapses between primary afferent C-fibres and lamina I projection neurones but not unidentified neurones in lamina I. The specific properties that enable projection neurones to undergo long-term potentiation and mediate hyperalgesia are not fully understood. Here, we have tested whether lamina I projection neurones differ from unidentified neurones in types or strength of primary afferent input and/or action potential-independent excitatory and inhibitory input. We used the whole-cell patch-clamp technique to record synaptic currents in projection and unidentified lamina I neurones in a transverse lumbar spinal cord slice preparation from rats between postnatal day 18 and 37. Lamina I neurones with a projection to the parabrachial area or the periaqueductal grey were identified by retrograde labelling with a fluorescent tracer. The relative contribution of NMDA receptors versus AMPA/kainate receptors to C-fibre-evoked excitatory postsynaptic currents of lamina I neurones significantly decreased with age between postnatal day 18 and 27, but was independent of the supraspinal projection of the neurones. We did not find a significant contribution of kainate receptors to C-fibre-evoked excitatory postsynaptic currents. Lamina I projection and unidentified neurones possessed functional GABAA and glycine receptors but received scarce action potential-independent spontaneous GABAergic and glycinergic inhibitory input as measured by miniature inhibitory postsynaptic currents. The miniature excitatory postsynaptic current frequencies were five times higher in projection than in unidentified neurones. The predominance of excitatory synaptic input to projection neurones, taken together with the previous finding that their membranes are more easily excitable than those of unidentified neurones, may facilitate the induction of synaptic long-term potentiation.

Activation of the parabrachio-amygdaloid pathway by immune challenge or spinal nociceptive input: a quantitative study in the rat using Fos immunohistochemistry and retrograde tract tracing

Peripheral nociceptive stimulation results in activation of neurons in the pontine parabrachial nucleus (PB) of rats. Electrophysiological studies have suggested that noxiously activated PB neurons project to the amygdala, constituting a potential pathway for emotional aspects of pain. In the present study we examined this hypothesis by combining retrograde tract tracing with Fos immunohistochemistry. Cholera toxin subunit B was injected into the amygdala of rats. After a minimum of 48 hours the rats were given a subcutaneous injection of 100 microl of 5% formalin into one hindpaw and killed 60-90 minutes later. A dense aggregation of retrogradely labeled neurons was seen in the external lateral PB. Fos-expressing neurons were present preferentially in the central, dorsal, and superior lateral subnuclei as well as in the lateral crescent area, as described previously. There was little overlap between the retrogradely labeled and Fos-expressing populations and double-labeled neurons were rare. In contrast, systemic immune challenge by intravenous injection of bacterial wall lipopolysaccharide resulted in a Fos expression that overlapped the retrograde labeling in the external lateral PB, and many double-labeled neurons were seen. While these data provide direct functional anatomical evidence that nociceptive information from the hindlimb is relayed to the amygdala via the parabrachial nucleus, the number of parabrachio-amygdaloid neurons involved is small. Considering the widespread activation of parabrachio-amygdaloid neurons by a variety of visceral and humoral stimuli, the parabrachio-amygdaloid pathway thus appears to be more involved in the mediation of information related to viscerally and humorally elicited activity than in transmission of spinal nociceptive inputs.

Development of adaptive behaviour of the respiratory network: implications for the pontine Kolliker-Fuse nucleus

Breathing is constantly modulated by afferent sensory inputs in order to adapt to changes in behaviour and environment. The pontine respiratory group, in particular the Kolliker-Fuse nucleus, might be a key structure for adaptive behaviours of the respiratory network. Here, we review the anatomical connectivity of the Kolliker-Fuse nucleus with primary sensory structures and with the medullary respiratory centres and focus on the importance of pontine and medullary postinspiratory neurones in the mediation of respiratory reflexes. Furthermore, we will summarise recent findings from our group regarding ontogenetic changes of respiratory reflexes (e.g., the diving response) and provide evidence that immaturity of the Kolliker-Fuse nucleus might account in neonates for a lack of plasticity in sensory evoked modulations of respiratory activity. We propose that a subpopulation of neurones within the Kolliker-Fuse nucleus represent command neurones for sensory processing which are capable of initiating adaptive behaviour in the respiratory network. Recent data from our laboratory suggest that these command neurones undergo substantial postnatal maturation.

Parabrachial-lateral pontine neurons link nociception and breathing

We investigated the role of the parabrachial complex in cutaneous nociceptor-induced respiratory stimulation in chloralose-urethane anesthetized, vagotomized rats. Noxious stimulation (mustard oil, MO) applied topically to a forelimb or hindlimb enhanced the peak amplitude of the integrated phrenic nerve discharge and, with forelimb application, increased phrenic nerve burst frequency. Bilateral inactivation of neural activity in the parabrachial complex with injection of the GABA agonist muscimol (3nl) markedly attenuated the response to MO application. Injection of the retrograde tracer FluoroGold within the medullary ventral respiratory column labeled neurons in dorsolateral pontine regions known to receive nociceptive inputs (i.e., Kolliker-Fuse, lateral crescent, and superior lateral subnuclei of the parabrachial complex). Extracellular recordings of 65 dorsolateral parabrachial neurons revealed about 15% responded to a noxious cutaneous pinch with either an increase or a decrease in discharge and approximately 40% of these exhibited a phasic respiratory-related component to their discharge. In conclusion, parabrachial pontine neurons contribute to cutaneous nociceptor-induced increases in breathing.

In cat four times as many lamina I neurons project to the parabrachial nuclei and twice as many to the periaqueductal gray as to the thalamus

The spinothalamic tract, and especially its fibers originating in lamina I, is the best known pathway for transmission of nociceptive information. On the other hand, different studies have suggested that more lamina I cells project to the parabrachial nuclei (PBN) and periaqueductal gray (PAG) than to the thalamus. The exact ratio of the number of lamina I projections to PBN, PAG and thalamus is not known, because comprehensive studies examining these three projections from all spinal segments, using the same tracers and counting methods, do not exist. In the present study, the differences in number and distribution of retrogradely labeled lamina I cells in each segment of the cat spinal cord (C1-Coc2) were determined after large wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) injections in either PBN, PAG or thalamus. We estimate that approximately 6000 lamina I cells project to PBN, 3000 to PAG and less than 1500 to the thalamus. Of the lamina I cells projecting to thalamus or PAG more than 80%, and of the lamina I-PBN cells approximately 60%, were located on the contralateral side. In all cases, most labeled lamina I cells were found in the upper two cervical segments and in the cervical and lumbar enlargements.

Afferent projections to nucleus reuniens of the thalamus

The nucleus reuniens (RE) is the largest of the midline nuclei of the thalamus and the major source of thalamic afferents to the hippocampus and parahippocampal structures. Nucleus reuniens has recently been shown to exert powerful excitatory actions on CA1 of the hippocampus. Few reports on any species have examined afferent projections to nucleus reuniens. By using the retrograde anatomical tracer Fluorogold, we examined patterns of afferent projections to RE in the rat. We showed that RE receives a diverse and widely distributed set of afferents projections. The main sources of input to nucleus reuniens were from the orbitomedial, insular, ectorhinal, perirhinal, and retrosplenial cortices; CA1/subiculum of hippocampus; claustrum, tania tecta, lateral septum, substantia innominata, and medial and lateral preoptic nuclei of the basal forebrain; medial nucleus of amygdala; paraventricular and lateral geniculate nuclei of the thalamus; zona incerta; anterior, ventromedial, lateral, posterior, supramammillary, and dorsal premammillary nuclei of the hypothalamus; and ventral tegmental area, periaqueductal gray, medial and posterior pretectal nuclei, superior colliculus, precommissural/commissural nuclei, nucleus of the posterior commissure, parabrachial nucleus, laterodorsal and pedunculopontine tegmental nuclei, nucleus incertus, and dorsal and median raphe nuclei of the brainstem. The present findings of widespread projections to RE, mainly from limbic/limbic-associated structures, suggest that nucleus reuniens represents a critical relay in the transfer of limbic information (emotional/cognitive) from RE to its major targets, namely, to the hippocampus and orbitomedial prefrontal cortex. RE appears to be a major link in the two-way exchange of information between the hippocampus and the medial prefrontal cortex.

Nociceptive excited and inhibited neurons within the pedunculopontine tegmental nucleus and cuneiform nucleus

The pedunculopontine tegmental nucleus (PPTg) is defined by its collection of cholinergic neurons surrounding the lateral portion of the superior cerebellar peduncle at the midbrain pontine junction. Antinociceptive functions have been attributed to the PPTg since electrical stimulation as well as injection of cholinergic agonists in this area produces analgesia. Nociceptive neurons have also been reported in the vicinity of the PPTg and cuneiform nucleus (CN). However, specific histochemical localization of nociceptive modulatory neurons has not been determined. Thus, the goal of this study was to classify neurons according to their response to a noxious stimulus and map their location based on staining of the cholinergic neurons in the PPTg. Extracellular microelectrode recordings were conducted in 19 male Sprague-Dawley rats under light halothane anesthesia. For each neuron identified, a series of noxious tail pinches were administered. The electrode tracts were marked with ionophoresis of pontamine blue. The location of 112 recorded neurons was determined on sections stained with NADPH diaphorase to identify the cholinergic boundaries of the PPTg. Neurons were classified into one of three cell types based on their consistent response to a noxious tail pinch (excited, inhibited, and non-responsive). Tail pinch excited neurons (n=16), inhibited neurons (n=10) and non-responsive neurons (n=23) were mapped within the cholinergic boundaries of the PPTg. Excited (n=9), inhibited (n=10) and non-responsive neurons (n=10) were also found more dorsally within the cuneiform nucleus. Thus, this study localizes nociception-responsive neurons to the region of the largely cholinergic PPTg, as well as the noncholinergic cuneiform nucleus.

Activation of the dorsal periaqueductal gray in the rat induces Fos-like immunoreactivity in select non-cholinergic mesopontine neurons

Autonomic responses evoked from the dorsal periaqueductal gray (dPAG) have been reported to be mediated in part by acetylcholine release in the medulla. To identify the possible origin of cholinergic neurons activated by dPAG stimulation, the pattern of Fos-like immunoreactivity (FLI) in the mesopontine cholinergic cell groups was examined in three groups of urethane anesthetized rats. Relative to surgery (n=6) and blood pressure control groups (n=6), chemical disinhibition of the dPAG (n=10) induced a significant increase in FLI in the lateral dorsal tegmental nucleus (LDTg) but not the pedunculopontine tegmental nucleus. LDTg neurons stained for choline acetyltransferase immunoreactivity however did not co-label for FLI. Other pontomesencephalic regions outside of the dPAG demonstrating a significant increase in FLI relative to controls included the lateral and ventrolateral columns of the PAG, the cuneiform nucleus, dorsal raphe, and the microcellular tegmental nucleus. These findings suggest that acetylcholine release in during dPAG stimulation does not originate from mesopontine neurons.

Alterations in dorsal horn neurones in a rat model of cancer-induced bone pain

Cancer-induced bone pain is a major clinical problem. A rat model based on intra-tibial injection of MRMT-1 mammary tumour cells was used to mimic progressive cancer-induced bone pain. At the time of stable behavioural changes (decreased thresholds to mechanical and cold stimuli) and bone destruction, in vivo electrophysiology was used to characterize natural (mechanical, thermal, and cold) and electrical-evoked responses of superficial and deep dorsal horn neurones in halothane-anaesthetized rats. Receptive field size was significantly enlarged for superficial neurones in the MRMT-1 animals. Superficial cells were characterised as either nociceptive specific (NS) or wide dynamic range (WDR). The ratio of WDR to NS cells was substantially different between sham operated (growth media alone) (26:74%) and MRMT-1 injected rats (47:53%). NS cells showed no significant difference in their neuronal responses in MRMT-1-injected compared to sham rats. However, superficial WDR neurones in MRMT-1-injected rats had significantly increased responses to mechanical, thermal and electrical (A beta-, C fibre-, and post-discharge evoked response) stimuli. Deep WDR neurones showed less pronounced changes to the superficial dorsal horn, however, the response to thermal and electrical stimuli, but not mechanical, were significantly increased in the MRMT-1-injected rats. In conclusion, the spinal cord is significantly hyperexcitable with previously superficial NS cells becoming responsive to wide-dynamic range stimuli possibly driving this plasticity via ascending and descending facilitatory pathways. The alterations in superficial dorsal horn neurones have not been reported in neuropathy or inflammation adding to the evidence for cancer-induced bone pain reflecting a unique pain state.

Organization of axonal projections from the anterolateral area of the bed nuclei of the stria terminalis

The anterolateral group of the bed nuclei of the stria terminalis (BSTalg) contains four distinct cell groups embedded within an undifferentiated anterolateral area (BSTal) that architectonically resembles a subjacent subcommissural zone (BSTsc). The overall distributions of axonal projections from various regions of the BSTal and from the BSTsc were determined with the Phaseolus vulgaris-leucoagglutinin (PHAL) anterograde tracer method and found to be identical. The BSTal and BSTsc share dense bidirectional connections, and also project heavily within the BST to the rhomboid and fusiform nuclei and the anteroventral and anterodorsal areas. They project less densely to the juxtacapsular, oval, magnocellular, ventral, and interfascicular BST nuclei. Outside the BST, brain areas receiving strong to moderate inputs from the BSTal and BSTsc fall into several functional groups: somatomotor system (nucleus accumbens, substantia innominata, ventral tegmental area, and retrorubral area and adjacent midbrain reticular nucleus), central autonomic control system (central amygdalar nucleus, dorsal lateral hypothalamic area, ventrolateral periaqueductal gray, parabrachial nucleus, and nucleus of the solitary tract), neuroendocrine system (paraventricular and supraoptic nuclei, hypothalamic visceromotor pattern generator network), and thalamocortical feedback loops (midline, medial, and intralaminar nuclei). The results indicate that the BSTal and BSTsc are parts of the same cell group (dorsal and ventral to the anterior commissure), which plays a role in coordinating visceral and somatic motor responses (during ingestive behaviors, for example), especially in response to noxious stimuli (learned anorexia associated with noxious stimuli). BSTal projections are distinct from those of the adjacent juxtacapsular, oval, fusiform, and rhomboid nuclei.

Lateral cervical nucleus projections to periaqueductal gray matter in cat

The midbrain periaqueductal gray matter (PAG) integrates the basic responses necessary for survival of individuals and species. Examples are defense behaviors such as fight, flight, and freezing, but also sexual behavior, vocalization, and micturition. To control these behaviors the PAG depends on strong input from more rostrally located limbic structures, as well as from afferent input from the lower brainstem and spinal cord. Mouton and Holstege (2000, J Comp Neurol 428:389-410) showed that there exist at least five different groups of spino-PAG neurons, each of which is thought to subserve a specific function. The lateral cervical nucleus (LCN) in the upper cervical cord is not among these five groups. The LCN relays information from hair receptors and noxious information and projects strongly to the contralateral ventroposterior and posterior regions of thalamus and to intermediate and deep tectal layers. The question is whether the LCN also projects to the PAG. The present study in cat, using retrograde and anterograde tracing techniques, showed that neurons located in the lateral two-thirds of the LCN send fibers to the lateral part of the PAG, predominantly at rostrocaudal levels A0.6-P0.2. This part of the PAG is known to be involved in flight behavior. A concept is put forward according to which the LCN-PAG pathway alerts the animal about the presence of cutaneous stimuli that might represent danger, necessitating flight. J. Comp. Neurol. 471:434-445, 2004.

A critical review of the role of the proposed VMpo nucleus in pain

The evidence presented by Craig and his colleagues for an important projection from lamina I spinothalamic tract neurons to a renamed thalamic nucleus (the posterior part of the ventral medial nucleus or VMpo), as well as to the ventrocaudal medial dorsal and the ventral posterior inferior thalamic nuclei, is critically reviewed. Of particular concern is the denial of an important nociceptive lamina I projection to the ventrobasal complex. Contrary evidence is reviewed that strongly favors a role of spinothalamic projections from both lamina I and deep layers of the dorsal horn to the ventrobasal complex and other thalamic nuclei and from there to the SI and SII somatosensory cortices in the sensory-discriminative processing of pain and temperature information.

A quantitative and morphological study of projection neurons in lamina I of the rat lumbar spinal cord

In the rat lumbar spinal cord the major supraspinal targets for lamina I projection neurons are the caudal ventrolateral medulla (CVLM), lateral parabrachial area (LPb) and periaqueductal grey matter (PAG). In this study we have estimated the number of lamina I neurons retrogradely labelled from each of these sites in the L4 segment, as well as the proportion that can be labelled by injecting different tracers into two separate sites. Our results suggest that this segment contains approximately 400 lamina I projection neurons on each side, and that approximately 85% of these can be labelled from either the CVLM or the LPb on the contralateral side. Around 120 lamina I cells in L4 project to the PAG, and over 90% of these cells can also be labelled from the CVLM or LPb. Most lamina I neurons projecting to CVLM or LPb are located in the contralateral dorsal horn, but in each case some cells were found to have bilateral projections. We also examined horizontal sections to investigate morphology and the expression of the neurokinin 1 (NK1) receptor in cells labelled from CVLM, LPb or PAG. There were no consistent morphological differences between these groups, however, while cells with strong or moderate NK1 receptor-immunostaining were labelled from LPb or CVLM, they seldom projected to the PAG. These results suggest that many lamina I cells project to more than one site in the brain and that those projecting to PAG may represent a distinct subclass of lamina I projection neuron.

Dendritic domains of nociceptive-responsive parabrachial neurons match terminal fields of lamina I neurons in the rat

This study investigates, in the anesthetized rat, the dendritic extent of parabrachial (PB) neurons whose nociceptive response to noxious stimuli has been previously recorded with an extracellular micropipette. The PB neurons were then injected with biocytin through the recording micropipette, via a juxtacellular technique. The dendritic arborization of individual PB neurons was carefully compared with the projections of medullary (trigeminal) and spinal lamina I neurons. The latter projections were labeled in separate animals that received injections of Phaseolus vulgaris-leucoagglutinin restricted to the superficial layers of spinal or medullary dorsal horn. We report here that: 1) PB neurons excited chiefly by noxious stimulation of the face have their dendritic tree located primarily within the field of lamina I trigeminal projections, i.e., in the caudal portion of PB area, around the external medial and the caudal part of the external lateral subnuclei; and 2) PB neurons excited chiefly by noxious stimulation of the paw or the tail have their dendritic tree located primarily within the field of lamina I spinal projections, i.e., in PB mid-extent, around the borderline between the external lateral and both the lateral crescent and the superior lateral subnuclei. Our results suggest the presence of an extensive excitatory axodendritic link between lamina I projections and PB nociceptive neurons around the lateral crescent and the external medial subnuclei. These findings strengthen the possibility of involvement of a subgroup of PB neurons in nociceptive processes.

Involvement of the parabrachial nucleus in thermogenesis induced by environmental cooling in the rat

The parabrachial nucleus (PBN), an integration site for autonomic regulation in the mesopontine brainstem, receives sensory information about cold temperatures from the skin. Here we investigated the role of the PBN in thermoregulation. Unilateral stimulation of the PBN (10-40 microA, 5 min) immediately increased the rate of O(2) consumption ( VO(2)) and heart rate, and the magnitude of these responses increased with the intensity of the stimulus in urethane-anesthetized rats. High-intensity (40 microA) stimulation of the PBN increased the temperature of the interscapular brown adipose tissue and that of the colon but had no effect on that of the tail skin, although weaker stimulation was without effect on these temperatures. Next, we examined the effects of bilateral lesioning of the PBN on environmental cooling-induced thermogenesis in conscious rats. Exposure of PBN-lesioned rats to a cool (16.6 degrees C) environment induced a significantly smaller increase in VO(2) than did that of rats with a sham operation or with lesions made outside of the PBN, and resulted in a marked decrease in body temperature in PBN-lesioned rats but not in other rats. Both frequency and duration of gross motor activity in the cool environment were similar between PBN-lesioned and sham-operated rats. These results suggest that the PBN is involved in the central mechanisms of cooling-induced thermogenesis.

A comparative reappraisal of projections from the superficial laminae of the dorsal horn in the rat: The forebrain

Projections to the forebrain from lamina I of spinal and trigeminal dorsal horn were labeled anterogradely with Phaseolus vulgaris-leucoagglutinin (PHA-L) and/or tetramethylrhodamine-dextran (RHO-D) injected microiontophoretically. Injections restricted to superficial laminae (I/II) of dorsal horn were used primarily. For comparison, injections were also made in deep cervical laminae. Spinal and trigeminal lamina I neurons project extensively to restricted portions of the ventral posterolateral and posteromedial (VPL/VPM), and the posterior group (Po) thalamic nuclei. Lamina I also projects to the triangular posterior (PoT) and the ventral posterior parvicellular (VPPC) thalamic nuclei but only very slightly to the extrathalamic forebrain. Furthermore, the lateral spinal (LS) nucleus, and to a lesser extent lamina I, project to the mediodorsal thalamic nucleus. In contrast to lamina I, deep spinal laminae project primarily to the central lateral thalamic nucleus (CL) and only weakly to the remaining thalamus, except for a medium projection to the PoT. Furthermore, the deep laminae project substantially to the globus pallidus and the substantia innominata and more weakly to the amygdala and the hypothalamus. Double-labeling experiments reveal that spinal and trigeminal lamina I project densely to distinct and restricted portions of VPL/VPM, Po, and VPPC thalamic nuclei, whereas projections to the PoT appeared to be convergent. In conclusion, these experiments indicate very different patterns of projection for lamina I versus deep laminae (III-X). Lamina I projects strongly onto relay thalamic nuclei and thus would have a primary role in sensory discriminative aspects of pain. The deep laminae project densely to the CL and more diffusely to other forebrain targets, suggesting roles in motor and alertness components of pain.

Responses of primate caudal parabrachial nucleus and Kölliker-fuse nucleus neurons to whole body rotation

The caudal aspect of the parabrachial (PBN) and Kölliker-Fuse (KF) nuclei receive vestibular nuclear and visceral afferent information and are connected reciprocally with the spinal cord, hypothalamus, amygdala, and limbic cortex. Hence, they may be important sites of vestibulo-visceral integration, particularly for the development of affective responses to gravitoinertial challenges. Extracellular recordings were made from caudal PBN cells in three alert, adult female Macaca nemestrina through an implanted chamber. Sinusoidal and position trapezoid angular whole body rotation was delivered in yaw, roll, pitch, and vertical semicircular canal planes. Sites were confirmed histologically. Units that responded during rotation were located in lateral and medial PBN and KF caudal to the trochlear nerve at sites that were confirmed anatomically to receive superior vestibular nucleus afferents. Responses to whole-body angular rotation were modeled as a sum of three signals: angular velocity, a leaky integration of angular velocity, and vertical position. All neurons displayed angular velocity and integrated angular velocity sensitivity, but only 60% of the neurons were position-sensitive. These responses to vertical rotation could display symmetric, asymmetric, or fully rectified cosinusoidal spatial tuning about a best orientation in different cells. The spatial properties of velocity and integrated velocity and position responses were independent for all position-sensitive neurons; the angular velocity and integrated angular velocity signals showed independent spatial tuning in the position-insensitive neurons. Individual units showed one of three different orientations of their excitatory axis of velocity rotation sensitivity: vertical-plane-only responses, positive elevation responses (vertical plane plus ipsilateral yaw), and negative elevation axis responses (vertical plane plus negative yaw). The interactions between the velocity and integrated velocity components also produced variations in the temporal pattern of responses as a function of rotation direction. These findings are consistent with the hypothesis that a vestibulorecipient region of the PBN and KF integrates signals from the vestibular nuclei and relay information about changes in whole-body orientation to pathways that produce homeostatic and affective responses.

The central autonomic nervous system: conscious visceral perception and autonomic pattern generation

The overall organization of the peripheral autonomic nervous system has been known for many decades, but the mechanisms by which it is controlled by the central nervous system are just now coming to light. In particular, two major issues have seen considerable progress in the past decade. First, the pathways that provide visceral sensation to conscious perception at a cortical level have been elucidated in both animals and humans. The nociceptive system runs in parallel to the pathways carrying visceral sensation from the cranial nerves and may be considered in itself a component of visceral sensation. Second, structures in the central nervous system that generate patterns of autonomic response have been identified. These pattern generators are located at multiple levels of the central nervous system, and they can be combined in temporal and spatial patterns to subserve a wide range of behavioral needs.

The projections of the midbrain periaqueductal grey to the pons and medulla oblongata in rats

It is now established that stimulation of the ventrolateral midbrain periaqueductal grey (PAG) evokes inhibition of nociceptive spinal neurons, which results in analgesia and a powerful attenuation of pain behaviour. It is postulated that the PAG exerts this inhibitory effect on spinal nociceptive functions through the activation of descending serotonergic and noradrenergic pathways that arise from the rostral ventromedial medulla (RVM) and pontine noradrenergic nuclei. To investigate the neuroanatomical substrate of this functional link between the PAG and RVM, as well as the pontine noradrenergic nuclei in the rat, we labelled axons that project from the ventrolateral PAG to various regions of the pons and medulla oblongata using the anterograde tracing substance, Phaseolus vulgaris leucoagglutinin. We demonstrated that some of PAG efferents really do terminate in the RVM and pontine noradrenergic nuclei, but a substantial proportion of them project to the intermediate subdivision of the pontobulbar reticular formation. Combining the axonal tracing with serotonin- and tyrosine-hydroxylase-immunohistochemistry, we also found that, in contrast to previous results, PAG efferents make relatively few appositions with serotonin- and tyrosine-hydroxylase-immunoreactive neurons in the RVM and pontine noradrenergic nuclei; most of them terminate in nonimmunoreactive territories. The results suggest that the ventrolateral PAG may activate a complex pontobulbar neuronal assembly including neurons in the intermediate subdivision of the pontobulbar reticular formation, serotonin- and tyrosine-hydroxylase-immunoreactive and nonimmunoreactive neurons in the RVM and pontine noradrenergic nuclei. This pontobulbar neural circuitry, then, may mediate the PAG-evoked activities towards the spinal dorsal horn resulting in the inhibition of spinal nociceptive functions.